If the screen normally turns off when you press half-shutter, like 600D... not sure what to do. Maybe call display_on in the print routine?

With 5D3, I have +20 on 50/1.8 at ~1 meter and +8 at ~3 meters. Both figures are quite repeatable.

I just tried your latest. That GUI you put on it is so sweet. Here are the results with my Sigma 35mm f/1.4 @ 1.5 meters so far:

My algorithm: +6, +6, +5, +5

Your algorithm: +6, +5, +5, +5

So they look to be equal. I'll try a few more lenses - if they're all about the same then it looks like your algorithm is good to go! This is so cool.

Edit: Here are my 50mm EF f/1.4 results:

My algorithm: +3, +2, +2, +3

Your algorithm: +1, +2, +2, +2